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Shoulders!

The shoulder joint (glenohumeral joint) is a ball and socket joint between the scapula and the humerus. It is the major joint connecting the upper limb to the trunk. It is one of the most mobile joints in the human body, at the cost of joint stability.


The shoulder joint is formed by the head of the humerus within the glenoid cavity (or fossa) of the scapula. Like most synovial joints, the articulating surfaces are covered with hyaline cartilage. The head of the humerus is much larger than the glenoid fossa, giving the joint a wide range of movement at the cost of inherent instability.


Joint Capsule and Bursae


The joint capsule is a fibrous sheath which encloses the structures of the joint. It extends from the neck of the humerus to the border of the glenoid fossa. The joint capsule is lax, permitting greater mobility (particularly abduction). The synovial membrane lines the inner surface of the joint capsule, and produces synovial fluid to reduce friction between the articular surfaces.


To reduce friction in the shoulder joint, several synovial bursae are present. A bursa is a synovial fluid filled sac, which acts as a cushion between tendons and other joint structures.


Ligaments


In the shoulder joint, the ligaments play a key role in stabilising the bony structures.

Coracohumeral ligament – attaches the base of the coracoid process to the greater tubercle of the humerus. It supports the superior part of the joint capsule.


Transverse humeral ligament – spans the distance between the two tubercles of the humerus. It holds the tendon of the long head of the biceps in the intertubercular groove.]

Coraco–clavicular ligament – composed of the trapezoid and conoid ligaments and runs from the clavicle to the coracoid process of the scapula. They work alongside the acromioclavicular ligament to maintain the alignment of the clavicle in relation to the scapula. They have significant strength but large forces (e.g. after a high energy fall) can rupture these ligaments as part of an acromio-clavicular joint (ACJ) injury. In severe ACJ injury, the coraco-clavicular ligaments may require surgical repair.


The other major ligament is the coracoacromial ligament. Running between the acromion and coracoid process of the scapula it forms the coraco-acromial arch. This structure overlies the shoulder joint, preventing superior displacement of the humeral head.


Movements


As a ball and socket synovial joint, there is a wide range of movement permitted:

Extension (upper limb backwards in sagittal plane) – posterior deltoid, latissimus dorsi and teres major.


Flexion (upper limb forwards in sagittal plane) – pectoralis major, anterior deltoid and coracobrachialis. Biceps brachii weakly assists in forward flexion.

Abduction (upper limb away from the midline in coronal plane):

The first 0-15 degrees of abduction is produced by the supraspinatus.

The middle fibres of the deltoid are responsible for the next 15-90 degrees.

Past 90 degrees, the scapula needs to be rotated to achieve abduction – that is carried out by the trapezius and serratus anterior.


Adduction (upper limb towards midline in coronal plane) – pectoralis major, latissimus dorsi and teres major.


Internal rotation (rotation towards the midline, so that the thumb is pointing medially) – subscapularis, pectoralis major, latissimus dorsi, teres major and anterior deltoid.

External rotation (rotation away from the midline, so that the thumb is pointing laterally) – infraspinatus and teres minor.


Mobility and Stability


The shoulder joint is one of the most mobile in the body, at the expense of stability. Here, we shall consider the factors the permit movement, and those that contribute towards joint structure.


Factors that contribute to mobility:

Type of joint – ball and socket joint.

Bony surfaces – shallow glenoid cavity and large humeral head

Inherent laxity of the joint capsule.


Factors that contribute to stability:

Rotator cuff muscles – surround the shoulder joint, attaching to the tuberosities of the humerus, whilst also fusing with the joint capsule. The resting tone of these muscles act to compress the humeral head into the glenoid cavity.


Glenoid labrum – a fibrocartilaginous ridge surrounding the glenoid cavity. It deepens the cavity and creates a seal with the head of humerus, reducing the risk of dislocation.

Ligaments – act to reinforce the joint capsule, and form the coraco-acromial arch.

Biceps tendon – it acts as a minor humeral head depressor, thereby contributing to stability.

Shoulder pain and injury are common. Your shoulder is the most mobile of all your joints. Just think about how much it can actually move.


The reason for this movement is a very small joint contact zone. This essentially means that your shoulder is quite unstable. That is why your shoulder muscles are so vital to a normally functioning shoulder.


In most cases, if you are suffering shoulder pain it is because your muscles are simply not strong enough or they are uncoordinated.


Most shoulder pain is caused by one of the following categories:

  • Rotator Cuff

  • Adhesive Capsulitis (frozen shoulder)

  • Bursitis Shoulder

  • Shoulder Dislocation (Instability)

  • Fractures

  • Shoulder Arthritis

Because of my passion for shoulders, I will be posting individual blog posts on these so that I can go into more detail!


References https://teachmeanatomy.info/upper-limb/joints/shoulder/ my favorite site for learning about the body!

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